You were looking for an outlet that was connected to the same breaker as the lights you have an issue with. It's common to have more than one set of lights connected to the same breaker but not necessarily any outlets. As an example, my house has things broken up with all lights and a few critical systems and outlets on one panel and everything else including most outlets on a second panel. The first one has a generator transfer switch.
I was suggesting that if you don't have any outlets connected to that breaker, you could find a socket (not necessarily on the offending switch) and convert it temporarily into an outlet for testing. If the outlet is turned off with the XTBM installed behind it, obviously that won't provide any diagnostics. If you plug the XTBM into a different set of lights and want to make sure those bulbs aren't going to cause issues themselves, you could remove them during testing.